1. Field of the Invention
The present invention relates to a wireless communication apparatus capable of directly communicating with another wireless communication apparatus and a communication method for the wireless communication apparatus.
2. Description of the Related Art
Transmission channels for data transfer include wireless transmission channels, and one common wireless transmission channel is a wireless LAN according to the IEEE 802.11 standard. The IEEE 802.11 standard defines two modes of wireless communication performed by wireless terminals. One mode is the infrastructure mode in which a plurality of wireless terminals communicates among themselves via an access point (AP) (also known as a base station). In this case, stations (STAs) connected to the base station communicate with each other under the control of the AP.
The other mode is the ad hoc mode, in which STAs directly communicate with each other without using a specific AP.
Moreover, wireless LAN standards include IEEE 802.11h as a supplementary specification for the MAC layer when operating a wireless LAN system using the 5 GHz frequency band. IEEE 802.11h specifies a function referred to as DFS (Dynamic Frequency Selection). The DFS function is a function for avoiding radio interference between a wireless LAN system using the 5 GHz band and a radar system using the same frequency band. Specifically, DFS includes a radar signal detection function and a function for stopping transmission of a signal on a frequency channel in use if a radar signal is detected.
As a technique related to the DFS function, Japanese Patent Laid-Open No. 2001-237847, for example, discloses a packet signal transmission device. The packet signal transmission device first measures the reception level of another system such as a radar system and compares the measured level with a threshold value preset in the packet signal transmission device. Then, depending on the result of the comparison, packet transmission is prohibited for a predetermined period of time. The abovementioned invention is a technique for prohibiting packet transmission in DFS and does not describe any technique for continuing communication by changing the frequency channel.
Meanwhile, Japanese Patent Laid-Open No. 2001-244851, for example, discloses a transmitting/receiving device for a frequency hopping communication method. According to this disclosure, when the transmitting/receiving device generates a hopping pattern that sequentially designates frequencies used for transmission/reception, the device stores an interfering frequency value of a frequency at which radio interference occurs, and uses the stored value afterward. Thus, in the case where there is a fixed interfering frequency, frequency hopping can be performed in such a manner that the interfering frequency is avoided.
The abovementioned invention is directed to a method for determining the frequency channel to be used subsequently when performing frequency hopping and does not contain detailed descriptions of procedures necessary to recreate a network.
Furthermore, Japanese Patent Laid-Open No. 2005-020162, for example, discloses a wireless communication apparatus that is applicable to a multihop wireless network in the ad hoc mode. In this disclosure, an STA that wishes to change the frequency channel sends a frequency channel change proposal containing the frequency channel to shift to and the time when the shift will take place by multihop transmission. Then, at the designated time of shift, all STAs simultaneously change the frequency channel. Thus, the frequency channel can be dynamically switched even in an ad hoc network.
However, radar systems are normally systems that may be used at any time. Moreover, IEEE 802.11h specifies that transmission should be stopped immediately after radar is detected.
Therefore, a method of simultaneously changing the frequency channel by time control, as with the abovementioned invention, is not suitable for the DFS function on a frequency channel where interference with a radar system may occur.
The following is a description of a common operation performed by an STA when interference with a radar system is detected in the case where the STA performs wireless communication in the infrastructure mode on a frequency channel where radio interference with a radar system may occur.
First, when an AP detects a radar signal, the AP stops wireless communication it is performing and recreates the network on a new frequency channel. An STA performing wireless communication via the AP detects the absence of the AP and searches for that AP. Specifically, the STA scans networks while successively switching frequency channels until the AP is found. When the STA finds the AP on the new frequency channel, the STA associates with the found AP. Thus, the STA can continue wireless communication via the AP with the frequency channel used being changed. That is to say, in the case where communication is performed in the infrastructure mode, if the DFS function of the AP operates properly, the communication network can be recreated on the changed frequency channel regardless of whether or not the STA is equipped with the DFS function.
Here, the case where the AP is not equipped with the DFS function or the case where the DFS function does not operate properly even though the AP is equipped therewith will be considered. In these cases, as long as the STA is equipped with the DFS function, the STA itself can stop transmission of a signal even when the AP does not detect radar interference.
However, in the case where the STA performs wireless communication with another STA via the same AP, the STA cannot resume communication with that other STA unless the AP recreates the network on the new frequency channel.